This invention relates to a method and means for finger joining random lengths of lumber, and more particularly to a method and machine structure for the continuous forming of vertical or horizontal impression type finger joints for lumber and for oriented or parallel grained plywood, referred to by the industry as "P.L.V."
Those skilled in the art are aware that horizontal finger joints can be made much more rapidly than vertical joints, due to a continuous forward motion which is accomplished with transfer chains, versus a stop and go operation-commonly used in the vertical finger joint system. However, this same transfer procedure is less reliable in producing uniform joints as opposed to the stop go system.
In a stop-go system, two pices of material are rigidly held by vise cages until the joint is completed and semi-cured. No misalignments can occur as is possible with transfer chains by which work pieces are indexed at various points for different operations. In addition, with the horizontal process, if a joint is misaligned when crowded through a radio frequency tunnel, pressure will occur unevenly in fingers and movement in the joint will also occur while curing. When moving along a tunnel the joints may become twisted which in turn may open part of the joint. Partial opening results in loss of pressure and weakens the glue line. The movement involved in transferring the lumber through the tunnel, unless it is held absolutely straight, weakens the glue line. In some cases, the glue line could fail completely. This results in marginal joints.
It is a well-known fact that pressure is required on the wood glue line in order to obtain a good structural bond. Neither the vertical or horizontal process can lay claim to a consistantly good glue bond on the outside fingers, but because of the configuration of a vertical joint, the loss of bondage on the outside fingers has less effect on the strength of the joint. With the horizontal finger joint process, an oversized piece can be joined and then resurfaced to eliminate the poorly bonded outside finger or outer fingers can be trimmed back to eliminate flare.
The common machined finger joint, be it vertical or horizontal, has flats at both the crest and root of the finger. This flat tip area takes up a sizable percentage of cross-section in the joint but contributes little or nothing to the strength of the joint. With the impression-type finger joint system, a fine pitch joint is used to hold the cross-section of the outside fingers to an absolute minimum. This fine pitch along with the knife-edged vertical finger tips are among the facts which make the impression-type joint superior to the more common, machined horizontal or vertical joint.
In summation, it is known that the efficiency of a joint is attributable to: Tensil strength of the material; alignment of the joint while it is being made and cured; joint preparation reliability that has the right length fingers and precise profile to give the proper glue line pressure; glue bond usually considered in terms of percent of wood failure; and percent of cross-section that has proper slope and pressure on the glue line.
The prior art lack of joint reliability is due to numerous factors, but primary reasons are misalignment and profile defects due to moisture content variations such as for instance, the differences in wood densities between spring and summer growth. A significant reason for this is the lack of a machine which could cycle form a joint quickly and yet achieve a strong joint as the random lengths of lumber proceed through the joint forming work station or stations. Additionally, prior art shaper saws have been large and for that reason require more than one arbor of shaper saws. The result of the double arbor of blades contributed to off tolerances in preshaped teeth. Other defects were to be found in the manner of applying adhesive to the joints which resulted in inconsistent application of the glue. It is difficult to join wet and dry lumber in radio frequency curing as the current is greater in the wet piece and thus can damage the glue line. In some cases, the glue itself has been of the wrong type. Reduced to readily understandable terms, the industry problems in finger joining of lumber come down to lack of machine speed, lack of simplicity in the operation thereof and lack of consistency in joint quality. The net result has been expensive finger jointed lumber with joints which are in important structural respects simply inadequate. Thus there has been recognized a need for a single cycled operation at one forming station at a speed which would make it commercially feasible to produce finger jointed lumber with consistently strong, reliable and durable joints.
Among the prior art references of interest are the following U.S. Pat. Nos.: 3,262,723; 3,942,233; 3,951,189; 3,985,169; 3,927,705 and 4,041,998.